Like most modern phones, the Xperia 5 IV's back cover is held on with a lot of fairly strong adhesive. You'll want a heat gun or hair dryer, a strong suction cup that's meant for removing LCDs, and non-marring prying tools such as guitar picks or plastic spudgers.

Heat the back of the phone evenly, being careful not to hold the heat gun in one place for too long. If you're using a heat gun and not a hair dryer the low heat setting should be all you need. You don't want to damage the battery or internal cables.

After about 30 seconds of using the heat gun, stick the suction cup to the back of the phone and see if the back cover is starting to come loose.

If it is, take your pick or spudger and carefully pry around the edges. You may need to continue heating the areas where the glue hasn't let go yet. You shouldn't need to insert the spudger or pick very far into the phone, the adhesive is only about 2 millimeters wide and there are connectors and cables you don't want to damage.